<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<script>
    var b;
    //undefined
    console.log("undefined:")
    if(b){
        console.log("true")
    }else
    {
        console.log("false")
    }
    //null
    b=null;
    console.log("null:")
    if(b){
        console.log("true")
    }else
    {
        console.log("false")
    }
    //0
    b=0;
    console.log("0:")
    if(b){
        console.log("true")
    }else    {
        console.log("false")
    }
    //NaN
    b=Number.NaN;//not a number
    console.log("NaN:")
    if(b){
        console.log("true")
    }else    {
        console.log("false")
    }
    //NaN
    b=Number.NaN;//not a number
    console.log("NaN  === NaN:")
    if(b==b){
        console.log("true")
    }else    {
        console.log("false")
    }
    //String ""
    b="";//not a number
    console.log("空串:")
    if(b){
        console.log("true")
    }else    {
        console.log("false")
    }

</script>
</body>
</html>